Enhanced alumina layer produced by CVD

ABSTRACT

The present invention introduces a new and refined method to produce α-Al 2 O 3  layers with substantially better wear resistance and toughness than the prior art. The α-Al 2 O 3  layer of the present invention is formed on a bonding layer of (Ti,Al)(C,O,N) with increasing aluminium content towards the outer surface. Nucleation of α-Al 2 O 3  is obtained through a nucleation step being composed of both aluminizing and oxidization steps. The α-Al 2 O 3  layer according to this invention has a thickness ranging from 1 to 20 μm and is composed of columnar grains. The length/width ratio of the alumina grains is from 2 to 12, preferably 5 to 9. The layer is characterized by a strong (012) growth texture, measured using XRD, and by the almost total absence (104), (110), (113) and (116) diffraction peaks.

RELATED APPLICATION DATA

This application is a divisional application of application Ser. No.10/431,505, filed on May 8, 2003, and claims priority under 35 U.S.C.§§119 and/or 365 to Swedish Application No. 0201417-3, filed May 8,2002, the entire contents of each of these documents is herebyincorporated by reference.

FIELD OF THE INVENTION

In the description of the background of the present invention thatfollows reference is made to certain structures and methods, however,such references should not necessarily be construed as an admission thatthese structures and methods qualify as prior art under the applicablestatutory provisions. Applicants reserve the right to demonstrate thatany of the referenced subject matter does not constitute prior art withregard to the present invention.

The present invention relates to a substrate of cemented carbide, cermetor ceramics onto which a hard and wear resistant coating is deposited.The coating is adherently bonded to the substrate and covers allfunctional parts thereof. The coating is composed of one or morerefractory layers of which at least one layer is a strongly texturedalpha-alumina (α-Al₂O₃).

BACKGROUND OF THE INVENTION

In the description of the background of the present invention thatfollows reference is made to certain structures and methods, however,such references should not necessarily be construed as an admission thatthese structures and methods qualify as prior art under the applicablestatutory provisions. Applicant reserves the right to demonstrate thatany of the referenced subject matter does not constitute prior art withregard to the present invention.

A crucial step in deposition of different Al₂O₃ polymorphs is thenucleation step. κ-Al₂O₃ can be grown in a controlled way on {111}planes of TiN, Ti(C,N) or TiC having the face-centered cubic structure.Transmission electron microscopy (TEM) has confirmed the growth modewhich is that of the close-packed (001) planes of κ-Al₂O₃ on theclose-packed {111} planes of the cubic phase with the followingepitaxial orientation relationships: (001)_(κ)//(111)_(TiX); [100]_(κ)//[112]_(TiX). An explanation and a model for the CVD growth of metastableκ-Al₂O₃ has been proposed (Y. Yoursdshahyan, C. Ruberto, M. Halvarsson,V. Langer, S. Ruppi, U. Rolander and B. I. Lundqvist, TheoreticalStructure Determination of a Complex Material: κ-Al₂O₃ , J. Am. Ceram.Soc. 82(6)1365–1380 (1999)).

When properly nucleated, κ-Al₂O₃ layers can be grown to a considerablethickness (>10 μm). The growth of even thicker layers of κ-Al₂O₃ can beensured through re-nucleation on thin layers of, for example, TiNinserted in the growing κ-Al₂O₃ layer. When nucleation is ensured theκ→α transformation can be avoided during deposition by using arelatively low deposition temperature (<1000° C.). During metal cuttinghigh temperature conditions may be created such that the κ→α phasetransformation has been confirmed to occur. In addition to phasestability there are several reasons why α-Al₂O₃ should be preferred formany metal cutting applications. α-Al₂O₃ exhibits better wear propertiesin cast iron, as discussed in U.S. Pat. No. 5,137,774. Further, a layerwhich has been nucleated as α-Al₂O₃ does not contain any transformationcracks and stresses. Nucleated α-Al₂O₃ should be more ductile thanα-Al₂O₃ formed totally or partially as a result of phase transformation,and even more ductile than κ-Al₂O₃, the plasticity of which is limitedby the defect structure.

However, a stable α-Al₂O₃ phase has been found to be more difficult tobe nucleated and grown at reasonable CVD temperatures than themetastable κ-Al₂O₃. It has been experimentally confirmed that α-Al₂O₃can be nucleated, for example, on Ti₂O₃ surfaces, bonding layers of(Ti,Al)(C,O), as shown in U.S. Pat. No. 5,137,774, or by controlling theoxidation potential using CO/CO₂ mixtures, as shown in U.S. Pat. No.5,654,035. The bottom line in all these approaches is that nucleationmust not take place on the 111-surfaces of TiC, TiN, Ti(C,N) orTi(C,O,N), otherwise κ-Al₂O₃ is obtained.

It should also be noted that in conventional prior art methods higherdeposition temperatures are usually used to deposit α-Al₂O₃. When thenucleation control is not complete, as is the case in many conventionalproducts, the resulting α-Al₂O₃ layers have, at least partly, beenformed as a result of the κ-Al₂O₃→α-Al₂O₃ phase transformation. This isespecially the case when thick Al₂O₃ layers are considered. These kindof α-Al₂O₃ layers are composed of larger grains withphase-transformation cracks. These coatings exhibit much lowermechanical strength and ductility than α-Al₂O₃ coatings that arecomposed of nucleated α-Al₂O₃.

The control of the α-Al₂O₃ polymorph on an industrial scale was achievedin the beginning of the 1990's with commercial products based on U.S.Pat. No. 5,137,774. In addition, α-Al₂O₃ has been deposited withpreferred coating textures. In U.S. Pat. No. 5,654,035 an alumina layertextured in the (012) direction, and in U.S. Pat. No. 5,980,988 analumina layer textured in the (110) direction are disclosed. In U.S.Pat. No. 5,863,640 a preferred growth either along (012), (104), or(110) directions is disclosed. U.S. Pat. No. 6,333,103 describes amodified method to control the nucleation and growth of α-Al₂O₃ alongthe (10(10)) direction.

SUMMARY OF THE INVENTION

The present invention provides a new, improved alumina layer where theα-Al₂O₃ phase is nucleated α-Al₂O₃ with a strong, fully controlled (112)growth texture.

According to one aspect, the present invention provides cutting toolinsert comprising a substrate having a surface at least partially coatedwith a coating, the coating having a total thickness of about 10–40 μm,one or more refractory layers of which at least one layer is an aluminalayer, the alumina layer being composed of columnar α-Al₂O₃ grains withtexture coefficients expressed as TC(hkl) of:

-   -   a) TC(012) and TC(024) both >1.8; and    -   b) TC(104), TC(110), TC(113) and TC(116) all <0.4;        wherein the texture coefficient TC(hkl) is defined as:

${{{TC}({hkl})} = {\frac{I({hkl})}{I_{0}({hkl})}\left\{ {\frac{1}{n}{\sum\frac{I({hkl})}{I_{0}({hkl})}}} \right\}^{- 1}}}\mspace{14mu}$where

-   -   I(hkl)=measured intensity of the (hkl) reflection    -   I₀(hkl)=standard intensity according to JCPDS card no 46-1212    -   n=number of reflections used in the calculation    -   (hkl) reflections used are: (012), (104), (110), (113), (024),        (116).

According to another aspect, the present invention provides a method offorming a coating, the method comprising: (i) forming a coating of(Ti,Al)(C,),N); and (ii) modifying the as-formed coating by applyingalternating aluminizing and oxidizing treatments thereto, whereby thecoating is provided with an aluminium content which increases toward thesurface thereof.

DETAILED DESCRIPTION OF THE INVENTION

It has been surprisingly found that α-Al₂O₃ layers having a strong,fully controlled (112) growth texture outperform conventional coatingswhich have random or other textures such as (011). The coating accordingthe present invention is essentially free from transformation stresses,having columnar α-Al₂O₃ grains with low dislocation density and withimproved cutting properties.

According to the present invention, a method has been found to controlthe nucleation step of α-Al₂O₃ so that a strong (012)-texture can beobtained. The method is characterized by the dominating presence of(012) texture coefficient and simultaneous absence of the other commonTCs typically found in conventional coatings.

This kind of an Al₂O₃ layer is especially suited for use in toughnessdemanding steel cutting applications such as interrupted cutting,turning with coolant and especially intermittent turning with coolant.The other area is cast iron where the edge strength of this kind ofalumina layer is superior to conventional cutting tools.

Before α-Al₂O₃ is deposited on a CVD or MTCVD-applied Ti(C,N) coating,several steps are needed. First, a modified bonding layer, such asdescribed in U.S. Pat. No. 5,137,774 (referred to as kappa-bonding inthis patent), is deposited on the Ti(C,N) layer and is characterized bythe presence of an Al concentration gradient. In addition, nitrogen gasis applied during deposition of this bonding layer. The aluminiumcontent on the surface of this layer being considerably, about i.e.−30%, higher than in the bonding layer according to U.S. Pat. No.5,137,774, and the bonding layer of the invention also containsnitrogen. The surface of this bonding layer is subjected to anadditional treatment(s) using a AlCl₃/H₂ gas mixture in order to furtherincrease the aluminium content. Subsequently, an oxidation treatment isperformed using a CO₂/H₂ gas mixture. The oxidation step is short andmay be followed by a short treatment with a AlCl₃/H₂ mixture, againfollowed by a short oxidization step. These pulsating or alternating(Al-treatments/oxidization) treatments create favorable nucleation sitesfor α-Al₂O₃. The growth of the alumina layer onto the surface modifiedbonding layer is started by sequentially introducing the reactant gasesin the following order: CO, AlCl₃, CO₂. The temperature shall preferablybe about 1000° C.

The present invention also relates to a cutting tool insert comprising asubstrate at least partially coated with a coating having a totalthickness of 20–40 μm, preferably 15–25 μm. The coating composed of oneor more refractory layers of which at least one layer is an alphaalumina layer. This α-Al₂O₃ layer is dense and defect-free, and iscomposed of columnar grains with a strong (012) texture. The columnargrains have a length/width ratio of from 2 to 12, preferably 5 to 9. Thecolumnar grains have a width of 0.5–2.5 μm, preferably 0.5–1.0 or1.5–2.5 μm. The total thickness of the alumina layer, or “S”, can be 2–5μm when the grains are 0.5–1.0 μm, and can be 5–15 μm when the grainsare 1.5–2.5 μm.

The texture coefficients (TC) for the α-Al₂O₃ according to thisinvention layer is determined as follows:

${{{TC}({hkl})} = {\frac{I({hkl})}{I_{0}({hkl})}\left\{ {\frac{1}{n}{\sum\frac{I({hkl})}{I_{0}({hkl})}}} \right\}^{- 1}}}\mspace{14mu}$where

-   -   I(hkl)=intensity of the (hkl) reflection    -   I₀(hkl)=standard intensity according to JCPDS card no 46-1212    -   n=number of reflections used in the calculation    -   (hkl) reflections used are: (012), (104), (110), (113), (024),        (116).

The texture of the alumina layer is defined as follows:

-   -   TC(012) and TC (024)>1.8, preferably 2.5–3.5, and simultaneously        TC(104), TC(110), TC(113), TC(116)<0.4, preferably <0.3.

It is noted that the intensities of the planes 012 and 024 are related.

The substrate preferably comprises a hard material such as cementedcarbide, cermets, ceramics, high speed steel or a superhard materialsuch as cubic boron nitride (CBN) or diamond, preferably cementedcarbide or CBN. When CBN is used, the substrate preferably materialcontains at least 40 vol-% CBN. In one preferred embodiment thesubstrate is a cemented carbide with a binder phase enriched surfacezone.

The coating comprises a first layer adjacent the substrate of CVDTi(C,N), CVD TiN, CVD TiC, MTCVD Ti(C,N), MTCVD Zr(C,N), MTCVDTi(B,C,N), CVD HfN or combinations thereof, and is preferably of Ti(C,N)having a thickness of from 1 to 20 μm, preferably from 1 to 10 μm. Anα-Al₂O₃ refractory layer is provided adjacent the first layer having athickness of from about 1 to 40 μm, preferably from 1 to 20 μm, mostpreferably from 1 to 10 μm.

According to one aspect, the refractory layer consists of α-Al₂O₃.According to another aspect, the refractory layer consists essentiallyof α-Al₂O₃, i.e.—the layer is mainly α-Al₂O₃, but may include minoramounts of impurities and/or other phases of α-Al₂O₃. According to yetanother aspect, the refractory layer comprises α-Al₂O₃.

According to one embodiment, there is also an intermediate layer of TiNbetween the substrate and said first layer with a thickness of <3 μm,preferably 0.5–2 μm.

In one embodiment the α-Al₂O₃ layer is the uppermost layer

In another embodiment there is another layer of carbide, nitride,carbonitride or carboxynitride of one or more of Ti, Zr and Hf, having athickness of from about 0.5 to 3 μm, preferably 0.5 to 1.5 μm atop theα-Al₂O₃ layer. This additional layer can have a thickness of from about1 to 20 μm, preferably 2 to 8 μm.

In yet another embodiment the coating includes a layer of κ-Al₂O₃ and/orγ-Al₂O₃, preferably atop the α-Al₂O₃, with a thickness of from 0.5 to 10μm, preferably from 1 to 5 μm.

Specific illustrative, but non-limiting, examples of the above will nowbe described.

EXAMPLE 1

Cemented carbide cutting inserts with a composition of 5.9% Co andbalance WC (hardness about 1600 HV) were coated with a layer of MTCVDTi(C,N). The thickness of the MTCVD layer was about 8 μm. On to thislayer 8 μm α-Al₂O₃ was deposited “coating a” according to the presentinvention as defined below, and conventional “coating b”. The insertswere studied by using XRD and the texture coefficients were determined.Table 1 shows the texture coefficients for the coating according to thisinvention and for the prior art coating. As can be seen a strong (012)texture is obtained according to this invention.

TABLE 1 hkl Invention (coating a) Conventional (coating b) 012 2.79 0.72104 0.05 0.64 110 0.19 1.63 113 0.07 0.87 024 3.26 1.16 116 0.15 0.97

Deposition Process of “Coating a” (Invention) is Specified as Follows

Step 1: MTCVD coating Gas mixture TiCl₄ = 4.0% CH₃CN = 1.0% N₂ =  20%Balance: H₂ Duration 250 min Temperature 850° C. Pressure 100 mbar Step2: Bonding layer Gas mixture TiCl₄ = 2.8% AlCl₃ = 0.8–4.2% CO = 5.8% CO₂= 2.2% N₂ = 5–6% Balance: H₂ Duration 60 min Temperature 1000° C.Pressure 100 mbar Step 3: Aluminizing step Gas mixture AlCl₃ = 0.8–4.2%Balance: H₂ Duration 15 min or 2 min pulsating Temperature 1000 C.Pressure 50 mbar Step 4: Oxidizing step Gas mixture CO₂ = 0.1% Balance:H₂ Duration 2 min or 20 s pulsating Temperature 1000 C. Pressure 100mbar Step 5: Nucleation step Gas mixture AlCl₃ = 3.2% HCL = 2.0% CO₂ =1.9% Balance H₂ Duration 60 min Temperature 1000° C. Pressure 210 mbarStep 6: Deposition Gas mixture AlCl₃ = 4.2% HCL = 1.0% CO₂ = 2.1% H₂S =0.2% Balance: H₂ Duration 420 min Temperature 1000° C. Pressure 50 mbar

EXAMPLE 2

The coatings a) and b) from the example 1 were tested with respect toedge toughness in longitudinal turning.

Work piece: Cylindrical bar Material: SS0130 Insert type: SNUN Cuttingspeed: 400 m/min Feed:  0.4 mm/rev Depth of cut:  2.5 mm Remarks: dryturning

The inserts were inspected after 2 and 4 minutes of cutting. As clearfrom Table 2 the edge toughness of the conventional product wasconsiderably enhanced when the coating was produced according to thisinvention.

TABLE 2 Flaking of the Flaking of the edge line (%) edge line (%) after2 minutes After 6 minutes Coating a (Invention) 0 <1 Coating b 38 62

EXAMPLE 3

The coating produced according to this invention was compared with amarket leader, referred to here as Competitor X. This coating iscomposed of MTCVD Ti(C,N) and α-Al₂O₃. X-ray diffraction analysis (XRD)was used to determine the texture coefficients for these competitorcoatings. Two inserts from Competitor X were randomly chosen for XRD.Table 3 shows the TCs obtained for the Competitor X. The coatings fromCompetitor X exhibit a (110) texture.

TABLE 3 hkl TC(hkl) 012 0.64 0.59 104 0.95 0.88 110 1.75 2.00 113 0.450.42 024 1.15 1.10 116 1.07 1.01

EXAMPLE 4

The inserts from the competitor X were compared with inserts producedaccording to the present invention with the same substrate compositionand the same coating structure. Before the tests the both insertsproduced according to this invention were examined by XRD. The strong(012) texture was confirmed.

Two inserts produced according to this invention were compared with twoCompetitor X inserts with respect to flank wear resistance in faceturning of ball bearing material

Work piece: Cylindrical tubes (ball bearings) Material: SS2258 Inserttype: WNMG080416 Cutting speed: 500 m/min Feed:  0.5 mm/rev Depth ofcut:  1.0 mm Remarks: Dry turning

Tool life criterion: Flank wear >0.3 mm, three edges of each variantwere tested.

Results: Tool life (min) Coating 1 22 (invention) Coating 2 23.5(invention) Competitor 1 15.5 (conventional) Competitor 2 13(conventional)

EXAMPLE 5

Cubic boron nitride (CBN) inserts containing about 90% ofpolycrystalline CBN (PCBN) were coated according to this invention andaccording to conventional coating techniques discussed in Example 1. Thecoated CBN was compared with uncoated CBN insert in cutting of steelcontaining ferrite. It is known that boron has a high affinity toferrite and diffusion wear occurs at high cutting speeds.

Work piece: Cylindrical bar Material: SS0130 Insert type: SNUN Cuttingspeed: 860 m/min Feed:  0.4 mm/rev Depth of cut:  2.5 mm Remarks: dryturning

Life time (min) Coated CBN (Invention) 22 Conventional coating 14Uncoated CBN 11

The described embodiments of the present invention are intended to beillustrative rather than restrictive, and are not intended to representevery possible embodiment of the present invention. Variousmodifications can be made to the disclosed embodiments without departingfrom the spirit or scope of the invention as set forth in the followingclaims, both literally and in equivalents recognized in law.

1. A method of forming a coating, the method comprising: (i) forming acoating of (Ti,Al)(C,),N); and (ii) modifying the as-formed coating byapplying alternating pulsating aluminizing and oxidizing treatmentsthereto, whereby the coating is provided with an aluminum content whichincreases toward the surface thereof and creating favorable nucleationsites for Al₂O₃.
 2. The method of claim 1, further comprising the stepof: (iii) applying at least one layer of α-A₂O₃ to the modified coating.